Anti-detonation device for a carburetor



y 14, 1957 H. H. DIETmciH ANTI-DETONATION DEVICE FOR A CARBURETOR FiledFeb. 15, 1954 IN V EN TOR.

Howard H DlfflC/I Ahomey v 25. 2:. 3 850m 3 mm 9 United States Patent OANTI-DETONATION DEVICE FOR A CARBURETOR' Howard H. Dietrich, Rochester,N. Y., assignor to General' Motors Corporation, Detroit, Mich., acorporation of Delaware Application February 15, 1954, SerialNo.419,284.

16 Claims." (Cl. 123-119 This invention pertains to means forpreventing. destructive detonation in internal combustion engines, andparticularly to means for automatically sensing incipient detonationand, thereafter, preventing the occurrence of destructive detonation.

If the present trend of increasingthe compression ratios of automobileengines continues, the problem of destructive detonation will become ofmajor importance. Detonation, as referred to hereinafter, is defined asthe conditions attendant with uncontrolled flame propagation afterignition, i. e. self-ignition. Ithas been'determined that heavydetonation for as long as five seconds could conceivably destroy anengine. Accordingly, itwould seem that the provision of some means forpreventing destructive detonation is necessary. Heretofore,.ithas-beensuggested that detonation could be prevented by manually enriching themixture ratio during-times of maximumengine output. This inventionrelates to means for automatically sensing incipient detonation, and,thereafter, automatically enriching the mixture ratio so as to precludedestructive detonation. Accordingly, among my objects are the provisionof means for sensing incipient detonation; the further provision ofmeans'for automatically enriching-the mixture ratio when theconditionsare such that detonation is likely to occur; andthe still furtherprovision of a carburetor including the' aforementioned anti-detonationmeans;

The aforementioned and other objects are accomplished in the presentinvention by providing means responsive to engine speed, or air flow,through the carburetor, for controlling the operation of the enrichmentmeans at predetermined throttle positions. Specifically, the deviceincorporates meanswithin the carburetor for enriching" the air fuelmixture, which means are controlled by four parameters, namely: throttleposition; intake passage mixture temperature; humidity of intakemixture; and either engine speed or airflow through the carburetor. Ithas been determined that detonation is a function of the aforementionedparameters in any given engine.

The specific mechanism includes a solenoid controlled fuel valve whichis disposed in the float chamber of a carburetor. The valve controls anorifice for admitting fuel to the carburetor venturi so as to enrich theair fuel mixture passing therethrough. The electric circuit forcontrolling energization of the solenoid valve includes a bridgecircuit, a throttle-position controlled switch, and a relay, havingelectrical connection with the bridge circuit, responsive tounidirectional unbalanced current flows therethrough.

Two legs of the bridge circuit are comprised by resistors of fixed ohmicvalue. A third leg includes a thermistor,

-the ohmic value of which varies with the temperature The fourth. leg ofi of intake manifiold air-fuel mixture. the bridge includes a non-linearvariable resistor, the ohmic value of which is controlled by eitherengine speed or the carburetor venturi pressure, and a variabler'e's'istor, the ohmic value of which changes with the moisture contentof the intake mixture.

The throttle controlled 2,791,995 Patented May 14, 1957 switch energizesthe bridge from a power source whenever the throttle valve is moved to,or past, a predetermined position. Accordingly, when the bridge circuitis unbalanced in one direction, i. c. sensing incipient detonation, therelay and solenoid will be energized, thereby enriching the air fuelmixture.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing wherein a preferred embodiment of the present invention isclearly shown.

In the drawing:

Fig. 1 is a schematic illustration of the anti-detonating device of thisinvention with the variable non-linear resistor being controlled byventuri depression.

Fig. 2 is a fragmentary view illustrating a modification wherein thevariable' non-linear resistor is controlled by a speed sensitive device.

With particular reference to Fig. 1, the invention is shown inconjunction with a carburetor 10 having a throttle controlled outletpassage 11. Accordingto conventional practice, the carburetor includes aventuri 12 and a float controlled fuel chamber 13. T he fuel chamber 1-3and the throat of the venturi 12 are interconnected by a nozzle and tube14 having a Y connection submerged within the fuel chamber. One leg 15of the Y carries a metered main fuelorifice 16 through which fuel may beadmitted from the fuel reservoir to the venturi threat, in theconventional manner. The other leg 17 contains a metered auxiliaryorifice 18'. However, communication between the orifice 18 and thefuel-chamber is controlled bya solenoid operated valve 19.

The solenoid operated valve 19 includes a plunger 20 which is biased bymeans of a spring 21 so that the valve is normally closed. A portion ofthe plunger 2%) is encompassed by a solenoid winding 22, which isconnected by a wire 23 to a. relay contact 24. The other end of thesolenoid winding 22 is connected to ground, as shown in Fig. 1. It willbe appreciated that whenever the solenoid winding 22 is energized, theplunger 20 will move to the right, asviewed in Fig. 1, whereupon metereddepression, or in other words, the airflow through the carburetor to theengine. As shown in Fig. l, the passage 25 is connected by a tube 26 toa suction motor 27. The suction motor 27 comprises a housing 28, theopen end of which is closed by a flexible diaphragm 29. The diaphragmconstitutes a one seat fora coiled spring 30, the other end of thespring 30 being seated in a cup-shaped member 31, which is carried by astud 32 having threaded connection with the housing 28. The spring 30exerts its force on the diaphragm 29 in a manner tending to move thediaphragm to the left, as viewed in Fig. l. The stud 32 may be rotatedso as to regulate the amount of force which spring 30 exerts upon thediaphragm 29. The tube 26 communicates with the interior of housing 28,thereby subjecting the inner surface of. diaphragm 29 to venturidepression, while the outer surface of the diaphragm 29 is subjected toatmospheric air pressure. The diaphragm 29' is connected to a rod 33,which is, in turn, connected through an adjustable coupling 34 to a rod35'.

'The' carburetor 10, in the usual manner, carries a pivotally supportedthrottle valve 36; The throttle valve 36 either aetuates directly, or isconnected by mechanical linkage37 toaswitch 38, the'arrangement beingsuch that the contacts of switch 38 are moved to the closed position andretained therein whenever the throttle valve 36 is moved to and beyond apredetermined angular posi tion. The particular angular position, atwhich the throttle valve 36 will close the switch 38 will, of course,vary with different engines. In the present invention, it will beassumed that the throttle valve 36 will close the switch 38 whenever thethrottle is opened 70, or more.

One contact 39 of the switch 38 is connected by a wire 40 to a directcurrent source 141, which may be constituted by an automobile storagebattery. The other contact 41 of the switch 38 is connected by a wire 42to a terminal 43. The terminal 43 forms part of a bridge circuitindicated generally by the numeral 100. The bridge 100 includes fourlegs, two of the legs including resistors of fixed equal, ohmic value 44and '45, respectively. The resistor 44 is connected between bridgeterminals 43 and 46, while the resistor 45 is connected between bridgeterminals 43 and 47. The bridge terminal 46 is connected by a wire 48through a rectifier 49, which may be either of the copper oxide orgermanium diode type, to a wire 50, which is connected with one end of arelay winding 51. The rectifier only permits current flow from bridgeterminal 47 to bridge terminal 46. The other end of the relay winding 51is connected by a wire 52 to bridge terminal 47. i

The third leg of the bridge 100 includes a thermistor S3. The ohmicvalue of the thermistor S3 varies with the temperature of the mixture inthe intake passage of the engine 160. Moreover, the thermistor ispreferably of the type having a negative temperature coefiicient, i. e.the ohmic value decreases as the temperature increases. One end of thethermistor S3 is connected to the bridge terminal 54 which is connectedto ground.

The fourth leg of the bridge 100 includes a resistor 55, the ohmic valueof which varies with the amount of moisture in the intake passagemixture. Specifically, the resistor 55 may be considered as beinghumidity responsive, in that its resistance decreases with an increasein the humidity of the intake air fuel mixture. The fourth leg of thebridge also includes a non-linear variable resistor 56, the movablecontact 57 of which is controlled by rod 35. The resistor 56 is showndivided and with an open circuit to illustrate its non-linearcharacteristic. The component resistors of the fourth bridge leg areconnected between bridge terminals 54 and 47, as shown in Fig. 1.

As those skilled in the art will be cognizant of, current will only flowbetween bridge terminals 46 and 47 when the bridge is unbalanced. Thatis, current will only flow when a potential difference exists betweenterminals 46 and 47. The non-linear variable resistor 56 permits mixtureenrichment in various ranges of air flow and prevents enrichment betweenthe aforementioned ranges. Thus, in conformity with engine knockcharacteristics, mixture enrichment is permitted between 1200 to 2400 R.P. M., prevented during intermediate engine speeds and permitted athigher engine speeds. The relay 51 includes a movable armature 58 havinga contact 59 adapted to be moved into engagement with contact 24,whenever the relay coil 51 is energized. The armature S8 is electricallyconnected by a wire 60 to bridge terminal 43, and, thence, through theswitch 38 to the D. C. power supply 141.

It has been determined that detonation in any given internal combustionengine is a function of four variables, namely: intake passage mixturetemperature; intake passage mixture humidity; throttle position; andeither engine speed, or air flow, through the carburetor or engine. Thepresent invention utilizes all four parameters for sensing incipientdetonation. Moreover, by employing a bridge 100, it will be appreciatedthat the operation of the sensing system will not change with variationin the electrical potential of the source 141.

With reference to Fig. 2, a modified embodiment of the present inventionis disclosed wherein engine speed,

as a parameter, has been substituted for venturi depres sion. In theembodiment of Fig. 2, the shaft is rotated by the engine, not shown. Theshaft has attached theretoan annular member 71 having pivotally attachedthereto flyweights 72 and 73. The flyweights 72 and 73 are alsopivotally interconnected with a yoke 74 by means of arms 75 and 76. Theyoke 74 is adapted for sliding movement along shaft 70, in response topivotal movement of the flyweights 72 and 73 about the member 71. Theyoke 74 receives a sliding rod 77, which, as shown, is connected to themovable contacts 57 of the variable non-linear resistor 56, which wouldbe connected at points A and B to points C and D, respectively, in thebridge circuit of Fig. 1.

Operation The anti-detonation device operates as follows. When theengine, not shown, is operating with the throttle valve 36 at a positionless than the predetermined position, i. e. 70 opening, the contacts 39and 41 of the switch 33 will. be separated, and, accordingly, the bridgesensing circuit will be deenergized. However, as soon as the throttlevalve 36 is moved to an open position of 70 or greater, the switch 38will be closed, thereby energizing the bridge 100. When the bridge isenergized, it may be either in a balanced or an unbalanced conditiondepending upon engine conditions. Thus,'the ohmic value of non-linearresistor 56 is controlled by the suction motor 27 in Fig. 1 and by theflyweights 72 and 73 in Fig. 2. Moreover, in either embodiment, theohmic values of the thermistor S3 and the humidity responsive resistor55 are determined by the semi-instantaneous condition of the fuelmixture in the intake passage. If the bridge is unbalanced with terminal47 having a greater positive potential than terminal 46, current willflow through the rectifier 49 and through the relay winding 51 to theterminal 46. In this manner, the relay winding 51 will be energizedwhere- 'upon contact 59 will be moved into engagement with contact 24,and solenoid 22 will be energized. Thus, the valve 19 will be moved toan open position, whereupon the mixture ratio passing through thecarburetor is enriched. These conditions prevail when the electricalbridge 100 senses incipient detonation. The enriched fuel mixture willhave an anti-knocking effect and tend to prevent excessive pressures inthe cylinders.

It should be noted that if the bridge 100 is unbalanced so that if point46 is at a greater positive potential than point 47, the relay winding51 will not be energized, inasmuch as the rectifier 49 only permitscurrent flow in the opposite direction between terminals 47 and 46.

The bridge circuit will sense incipient engine detonation in thefollowing manner, if the several parameters are considered individually.The thermistor 53, which is responsive to the temperature of the intakepassage mixture, will sense incipient detonation, or knock, and as thetemperature increases, the ohmic value of the thermistor decreases.Thus, the bridge circuit 100 will be affected by a temperature increasesince the potential of terminal 46 will approach ground potential. Inthis manner, current will flow from terminal 47 to terminal 46, whichcurrent flow is permitted by the rectifier 49, and the relay 51 will beenergized. Similarly, as venturi pressure decreases with increasedengine speed, the diaphragm 29 will move to the right, in Fig. 1, andthe yoke '74 will move to the right, in Fig. 2, with an increase inengine speed. Accordingly, additional resistance 56 will be introducedinto the bridge leg between terminals 47 and 54 as engine speedincreases. This will have the desired effect of increasing the potentialof terminal 47 thereby causing current flow in the same direction asthat of an increase in temperature on the thermistor S3.

The arm 57 of the resistor 56 will be in the open circuit position ofthe mono-linear resistor 56 between engine speeds of 1200 to 2400 R. P.M., thereby introducing an infinite resistance in the bridge leg betweentermipals-47 and 54to cause'a current flowbetween'terminals 47 and 46irrespective of the other parameters of detonation. Thus, when-the-arm57 is in the open circuit position, the relay 51 will be energized andthe mixture will be enriched if the throttle is in a position whereswitch 38 is closed.

Thehumidity responsive resistor 55 decreases in ohmic value as thehumidity of the intake passage mixture increases. Thus, if thehumidity'of the intake-passage mixture is high, the resistor 55 mayprevent an unbalanced bridge condition even though the mixturetemperature or engine speed is in the critical range, since highhumidity tends to prevent detonation or engine knocking. More over, atengine speeds above 2400 R. P. M., the ohmic value of the resistance 56introduced into bridge leg between terminals 47 and 54 increasesproportionally.

From the aforegoing it is manifest that the present invention providesautomatic means for sensing incipient detonation, which means areautomatically operable to effect enrichment of the combustible mixture.

While the embodiment of the present invention as herein disclosed,constitutees a preferred form, it is 'to be understood that other formsmight be adopted.

What is claimed is as follows:

I. The combination with an internal combustion en-, gine having anintake passage and a carburetor, said carburetor having a mixturepassage formed with a venturi, a throttle valve in said mixture passagefor controlling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed insaid venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid responsive to incipient engine detonation, andmeans conditioning the energizing circuit for operation when saidthrottle valve is at a predetermined position.

2. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi-and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel-orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaidauxiliary orifice, a solenoid for opening said valve so as to enrichthe mixture'flowingthr'oiigh said mixture passage, an energizing circuitfor said "solenoid including a relay, an electrical circuit responsiveto incipient engine detonation and'controlling the energization of saidrelay, and means for conditioning said electrical circuit for operationwhen said throttle valve is at a predetermined open position.

3. The combination set forth in claim 2 wherein the last recited meanscomprises a throttle valve controlled switch.

4; The combination set forth in claim 2 wherein the sensing meanscomprises a bridge circuit.

5. The combination set forth in claim 2 wherein said sensing circuitcomprises a bridge circuit, and wherein said relay is electricallyconnected across said bridge so as to be energized by a certainunbalanced condition thereof.

6. The combination set forth in claim 5 wherein-said bridge circuitincludes a variable resistance element, the ohmic value of which iscontrolled by venturi depression.

7'. The combinationwith. aniriternal combustion engine having an intakepassage and a carburetor, said carcommunication between the mixturepassage and the intake passage, a fuel reservoir, a fuel nozzle disposedin said vent'uri and a main fuel orifice for admitting fuel from saidreservoir to said nozzle; of means for preventingdestructive detonationin said engine comprising,

an auxiliary fuel orifice for admitting fuel to said nozzle, a valveforcontrolling communication between said fuel reservoir and saidauxiliary orifice, a solenoid for opening said valve so as to enrich themixture flowing through said mixture passage, an energizing circuit forsaid solenoid including a relay, an electrical circuit responsive toincipient engine detonation for controlling the energization' of saidrelay, said circuit comprising a bridge circuit, said relay beingelectrically connected across said bridge so as to be energized by acertain unbalanced condition thereof, and means for conditioning saidbridge circuit for operation when said throttle valve is at apredetermined open position, said bridge circuit including a variableresistance element, the ohmic value of which is the intake passage, afuel reservoir, a fuel nozzle disposed in said venturi and a main fuelorifice for admitting fuel from said reservoir to said nozzle; of meansfor preventing destructive detonation in said engine comprising, anauxiliary fuel orifice for admitting fuel to said nozzle, a valve forcontrolling communication between said fuel reservoir and said auxiliaryorifice, a

solenoid for opening said valve so as to enrich the mixture flowingthrough said mixture passage, an energizing circuit for said solenoidincluding a relay, an electrical circuit responsive to incipient enginedetonation for controlling the energization of said relay, said circuitcomprising a bridge circuit, said relay being electrically connectedacross said bridge so 'as to be energized by a certain unbalancedcondition thereof, and means for conditioning said'bridge circuit foroperation when said throttle valve is at a predetermined open position,said bridge circuit including a plurality of resistors, the ohmic valueof one resistor varying wth the temperature of the mixture in the intakepassage of said engine, the ohmic value of another resistor beingdependent upon the humidity of the mixture in said intake passage.

9.The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, on energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by 'a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including aplurality of resistors, the ohmic value of one resistor being controlledby engige speed, the 'ohmic va'lue of another resistor varying with thetemperature of the mixture in the engine intake passage.

10. The combination with an internal combustion engine having an intakepassage and a carburetor, said 'carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including aplurality of resistors, the ohmic value of one resistor varying withengine speed, the ohmic value of another resistor being dependent uponthe humidity of the mixture in the engine intake passage.

11. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including aplurality of resistors, the ohmic value of one resistor being controlledby venturi depression, the ohmic value of another resistor varying withthe temperature of the mixture in the engine intake passage.

12. The combination with an internal combustion engine having an intakepassage and a carburetor, said i carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage for controlling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication bebridge circuit including aplurality of resistors, the ohmic value of one resistor being controlledby venturi depression, the ohmic value of another resistor dependingupon the humidity of the mixture in said engine intake passage.

13. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including aplurality of resistors, the ohmic value of one resistor being controlledby engine speed, the ohmic value of a second resistor varying with thetemperature of the mixture in the engine intake passage, and the ohmicvalue of a third resistor depending upon the humidity of the mixture insaid intake passage.

14. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including aplurality of resistors, the ohmic value of one resistor being controlledby venturi depression, the ohmic value of a second resistor varying withthe temperature of the mixture in the engine intake passage, and theohmic value of a third resistor depending upon the humidity of themixture in said intake passage.

15. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge circuit including avariable resistance element, the ohmic value of which is dependent uponthe humidity of the mixture in said engine intake passage.

16. The combination with an internal combustion engine having an intakepassage and a carburetor, said carburetor having a mixture passageformed with a venturi, a throttle valve in said mixture passage forcontrolling communication between the mixture passage and the intakepassage, a fuel reservoir, a fuel nozzle disposed in said venturi and amain fuel orifice for admitting fuel from said reservoir to said nozzle;of means for preventing destructive detonation in said enginecomprising, an auxiliary fuel orifice for admitting fuel to said nozzle,a valve for controlling communication between said fuel reservoir andsaid auxiliary orifice, a solenoid for opening said valve so as toenrich the mixture flowing through said mixture passage, an energizingcircuit for said solenoid including a relay, an electrical circuitresponsive to incipient engine detonation for controlling theenergization of said relay, said circuit comprising a bridge circuit,said relay being electrically connected across said bridge so as to beenergized by a certain unbalanced condition thereof, and means forconditioning said bridge circuit for operation when said throttle valveis at a predetermined open position, said bridge including a variableresistance element, the ohmic value of which is dependent upon thetemperature of the mixture in said engine intake passage.

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